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Abstract: The degradation of two pesticides, bromoxynil and trifluralin, was investigated in ultrapure and natural water solutions under ultraviolet (UV) light and a combination of UV and hydrogen peroxide (H2O2). The effect of pH on the photooxidation of the pesticides was also studied. The results indicated that under direct photolysis with monochromatic light at 253.7 nm and different conditions, the photochemical rates followed first-order kinetics, with fluence-based rate constants ranging from 9.15 × 10−4 to 6.37 × 10−3 cm2 mJ−1 and 7.63 × 10−3 to 1.47 × 10−2 cm2 mJ−1 for bromoxynil and trifluralin, respectively. Quantum yields, in the range of 0.08–0.25 for bromoxynil and 0.12–0.72 for trifluralin, were observed in experiments using ultrapure water. The study also found that the UV/H2O2 process enhanced the oxidation rate in comparison to direct photolysis. A 90% degradation with UV dose of 333 and 188 mJ cm−2 was achieved for bromoxynil and trifluralin, respectively, in natural water, in presence of 8.8 × 10−4 M H2O2. To assess the aquatic toxicity, the Microtox® 81.9% screening test protocol was used before and after treatment. The test results indicated a decrease in the acute toxicity of the samples after treatment for both pesticides. [Copyright &y& Elsevier]